Ammonium nitrate composite propellant and method of preparation



United States Patent AMMONIUM NITRATE COMPOSITE PROPELLANT AND METHOD OF PREPARATION Barnet R. Adelman, Waco,Tex., assignor to Phillips Petroleum Company, a corporationof Delaware No Drawing. Application August 8, 1 955 Serial No. 527,133

12 Claims. (Cl. 52-.5)

large quantities and thus provides an attractive source of oxidizer component for rocket propellant compositions. Aparticularly successful rocket propellant composition comprises from 50 to 90 weight percent ammonium nitrate with the remainder being made up of a rubberycopolymer of a diene such as 1,3-butadiene and a heterocyclic nitrogen compound such as 2-methyl-S-vinylpyridine together with suitablecuring agents. The curing agents can be a quaternization agent such as an alkyl halide or ,can be a vulcanizing agent such as sulfur.

.These propellant compositions and a method for their manufacture aremore fully described in copending application Serial No. 284,447, filed April 25, 1952, by W. B. Reynolds and J. E. Pritchard.

Propellant compositions must be capableof passing rigorous test procedures so as to insure satisfactory operation after extended periods of storage and under different conditions of temperature. Methods have been provided for preventing the entrance of moisture into the finished propellant in storage' and various precautions are ordinarily taken to prevent deterioration of the propellants prior to their intended use. Ammonium nitrate has the ability to exist in several different crystalline forms and these different crystalline forms exist at difierent temperatures. Ammonium nitrate undergoes a volume change of about 3.8 percent in the course of a phase change which occurs at about 90 F. Phase III, which is stable above 90 'F., has lower density than phase IV, which is stable below 90 F. e The propellant compositions are usually cured at a temperature higher than 90 F. but are usually stored prior to their use at a temperature under 90 F. andtherefore it is usually inevitable that the ammonium nitrate in the propellantjwill go through this phase change during or after its preparation.

Discounting the phase-change region, the rubberbinder and the oxidizer have thermal coefficients of expansion of the same order of magnitude (2x10- inches/inches/ F.). The 90 F. phase change which produces the volume change is roughly equal to that produced by a temperature change of 110 'F., however, the ammonium nitrate can undergo this volume change over a temperature pressible, greater strains are necessarily imposed both upon the ammonium nitrate crystal and the rubbery 5 cells of binder material. When the temperature of the binder, which is in the form of a thin-film surrounding the ammonium nitrate crystal.

I have discovered that the deleterious effects of the change in volumeof ammonium nitrate resulting from the changes from phase IV to phase III can be avoided by maintaining the ammonium nitrate at a temperature between about F. and about 200 F. during drying of the ammonium nitrate, between about 90 F. and about F. during the step of incorporation of the ammonium nitrate and binder, and between about F. and about F. during the curing period. Thus, the ammonium nitrate and binder are incorporated into a propellant composition under such conditions that all of theammonium nitrate exists in phase III. When the resulting propellant is cooled to a temperature below 90 F. for a time sufiicient to effect transition from phase III to phase IV, the ammonium nitrate particles, encased in rubbery binder, decrease in volume and exist as free-floating particles so that no stresses are placed on the binder material. When the propellant is again heated to a temperature above 90 for a period of time sufiicient to cause reversion of the ammonium nitrate from phase IV to phase III, the ammonium nitrate particle within its rubbery case regains its original volume and no stresses are placed on the rubbery binder because the volume of the ammonium nitrate is the same as when the propellant composition was cured.

It is believed that the method of this invention avoids the formation of a metastable crystal phase, which results in the ammonium nitrate passing directly from phase II to phase IV without passing through phase III. Thus, according to the process of this invention the ammonium nitrate is incorporated into the priormaterial with the ammonium nitrate crystals being in phase III and at their greatest specific volume.

I have also found that it is advantageous to incorporate a slip agent with the ammonium nitrate-rubbery binder composition so as to prevent a bond between the ammonium nitrate and the rubbery bindery material. The slip agent can be a mold-release material or parting agent such as tricalcium phosphate, milori blue or lubricants such as micro-crystalline wax, petrolatum and, tricresyl phosphate. High energy compounds such as milori blue, tricalcium phosphate and tricresyl phosphate are generally preferred since their presence usually adds to the performance characteristics of the resulting propellants. The slip agent is generally added to the ammonium nitrate prior to incorporation of the ammonium nitrate with the binder material and is preferably utilized in an amount of 0.01 to 5.0 parts by weight per 100 parts of combined oxidizer and binder.

.The incorporation of a slip agent, according to the process of this invention, in the propellant composition is believed to create a cellular structure of binder material containing the ammonium nitrate encased Within the cured propellant is reduced so that the ammonium nitrate passes from the phase III to phase IV, the ammonium nitrate particles are contained in cells of binder material as free-floating particles. The presence of the slip agent facilitates detachment of the ammonium nitrate crystals from the rubber binder material during these phase changes.

I have found that ammonium nitrate propellants prepared according to the process of this invention are free from voids, cracks and 'fissuresL' Such imperfections have been common prior to the practice of the method of this invention and are believed to be a result of fracture of the propellant grain resulting from change of phase as a result of temperature change. Propellants prepared according to the methodof this invention have provided superior performance after periods of thermal cycling and have provided substantially constant values of temperature coefiicients of steady state pressure in tests conducted after periods of thermal cycling.

The following examples will illustrate the invention butare not to 'be construed as limiting the invention.

EMMPLE I Propellant charges were prepared having compositions as set forth in Table I.

Table 1 Parts by weight Propellant compositions were prepared using 17.5 parts by weight of the above binder composition, 82.5 parts "by weight of ammonium nitrate oxidizer and 0.25 part by weight of milori blue. This propellant composition isdesignated as propellant A.

A second propellant was prepared having the same composition as propellant A but including in addition 0.41 part by weight of tricalcium phosphate. This propellant is designated as propellant B.

These propellant compositions were prepared according to the following procedure. The copolymer was dried in 100 lb. batches using a size 12 Baker-Perkins mixer. The Mooney range for the dried material was set at 20 to 30. The carbon black was mixed with the copolymer during the drying procedure.

The ammonium nitrate oxidizer was dried in 300 lb. batches, first in a rotary kiln predryerand then in a Stokes rotary vacuum drier for a period of about 6 hours at 200 F. The oxidizer was then cooled in a rotary kiln cooler to a temperature between 90 and 1.60" F. The oxidizer was then ground in a one SH-micropulverizer to the desired degree of fineness. The finely ground ammonium nitrate was then thoroughly blended with the milori blue.

The carbon black-'copolymer masterbatch was mixed in a Banbury mixer until the temperature of the masterbatch reached 130 F., after which the remaining components of the binder composition were added, with the mixer stopped, and then mixing was continued for 2 minutes. About 30 percent of the oxidizer and milori blue blend and about 30 percent of the TP-90B were added and mixing continued for one or two minutes, after which the remaining oxidizer and milori blue blend and TP-0B were added and mixing was continued until total incorporation was achieved as evidenced by increased mixer power requirements. The mixing was continued for one minute after total incorporation was achieved.

The propellant composition was then extruded, cut in desired lengths and cured for 24 hours at 180 F.

The cured propellant grains were free from pin holes, fissures and other signs of imperfection after being cooled to room temperature.

Some of the burning characteristics of propellant A and propellant B are shown in the following Table II.

4 EXAMPLE n Tricalcium phosphate was tried as an agglomeration inhibitor. A 1b. bag of prilled, undried ammonium nitrate was blended with 0.5 lb. of tricalcium phosphate and stored at ambient temperature for 40 days. Simultaneously a 10 lb. sample of dried, ground ammonium nitrate was blended with 0.05 lb. of tricalcium phosphate and similarly stored. Both samples remained free flowing and unagglomerated whereas untreated samples of the same materials were found to be agglomerated after identical storage conditions.

Reasonable variations and modifications are possible within the scope of the disclosure of the present invention, the essence of which is an improved process for preparing rubber-base, ammonium nitrate propellants which comprises drying the ammonium nitrate at a temperature below the temperature at which a metastable phase is formed; incorporating the ammonium nitrate into the rubber binder; and curing the composition under temperature conditions so that the ammonium nitrate is maintained in phase III. The utilization of'a slip agent provides an added advantage in the process.

That which is claimed is:

1. A process for the production of a solid rocket propellant charge comprising from about 50 to about 90 weight percent ammonium nitrate and'from about 10 to about 50 weight percent of a rubber binder which comprisesdrying the ammonium nitrate -at atemperaturc above 90 F. and not substantially higher than 200 F.;

incorporating the dried ammonium nitrate into substantially dry, uncured rubber at a temperature in therange about 90 and F.; forming the resulting mixture into charges; and curing the charges at a temperature in the range of about F. to about P. so that the ammonium nitrate is maintained at a temperature in the range of 90 to 200 F. from the drying step to the end of the curing step.

2. A process according to claim 1 wherein tricresyl phosphate is incorporated into the propellant charge so as to prevent a bond between the ammonium nitrate and the rubber binder.

3. A process according to claim 1 wherein milori blue is incorporated into the propellant charge so as to prevent a bond between the ammonium nitrate and the rubber binder.

4. A process according to claim 1 where micro-crystalline wax is incorporated into the propellant charge so as to prevent a bond between the ammonium nitrate and the rubber binder.

5. A process according to claim 1 wherein a-slip agent selected from the group consisting 'of tricalcium phosphate,

miloriblue, micro-crystalline wax, petrolatum, and tricresyl phosphate is incorporated into the propellant charge so as to prevent a bond between the ammonium nitrate and the rubber .binder.

.6. A process according toclaim 5 wherein the slip agent is tricalcium phosphate.

7. In the process for the production of a solid rocket propellant comprising from 50 to 90 weight percent ammonium nitrate and from 10 to 50 weight percent of a rubber binder the improvement comprising the steps of drying the ammonium nitrate at a temperature above 90 F. and not substantially higher than about.200 F.; im-

.mediately incorporating the dried ammoniumnitrate into the substantially dry, uncured rubber at a temperature in the range of about 90 to 160 F.;.immediately forming the resulting mixture into charges; and immediately curing the charges at a temperaturein'the range of about 175 to about 180 F. so that the ammonium nitrate is maintained in the formof phase 111' during all of the steps of producing the solid rocket propellant.

8. In the process 'for the production of solid rocket propellant grains which comprises incorporating a major ampunt of ammonium'nitrate into a minor amount of the copolymer of a conjugated diene having up to 8 carbon atoms per molecule and at least one CH3=CR substituted heterocyclic nitrogen base selected from the group consisting of pyridine, quinoline, an alkyl substituted pyridine and an alkyl substituted quinoline, wherein the total number of carbon atoms in the nuclear alkyl substituent is not more than twelve and wherein R is selected from the group consisting of hydrogen and an alkyl radical and where said 7 GHQ-#0411 group is. attached to a nuclear carbon atom, forming the I mixture into grains, and curing the grains, the improvement which comprises converting the ammonium nitrate into the form of phase III prior toincorporation into the copolymer by maintaining the ammonium nitrate at a.

temperature in the range of about 90 to about 200 F. 1

and maintaining the ammonium nitrate in the temperature range of about 90 to about 200 F. during all of the steps of producingthe propellant grains 9. In the process for the production of solid rocket propellant grains comprising incorporating from 50 to 90 weight percent of ammonium nitrate into 10 to weight percent of a rubbery copolymer of a 1,3-butadiene and'2-methyl-5-vinylpyridine, forming the mixture into "grains, and curing the grains, the improvement which comprises converting the ammonium nitrate into the form of phase III prior to incorporation into the rubbery copolymer by maintaining the ammonium nitrate at a temperature in the range of about 90 to about 200 F. and maintaining the ammonium nitrate in the temperature range of about 90 to about 200 F. during all of the steps of producing the propellant grains. v

10. A solid propellant grain consisting essentially of about 50 to about 90 weight percent of ammonium nitrate and from about 10 to about 50 weight percent of rubber wherein the grain is prepared by drying the ammonium nitrate at a temperature in the range between and 200 F. so that the ammonium nitrate exists in the form of phase III; incorporating the dried ammonium nitrate with substantially dry, uncured rubber at a temperature in the range of about 90 to F.; forming the resulting mixture into a grain; and curing the grain at a temperature in the range of about to about F. so that the ammonium nitrate is maintained in the form of phase III during the entire operation of preparing the grain.

11. A solid propellant grain consisting essentially of 50 to 90 weight percent ammonium nitrate and from 10 to 50 weight percent of a rubbery copolymer of 1,3-butadiene and 2-methyl-5-vinylpyridine wherein the grain is prepared by drying the ammonium nitrate at a tempera- ;ture in the range between 90 and 200 P. so that the ammonium nitrate exists in the form of phase III; incorporating dried ammonium nitrate into substantially dry, uncured rubbery copolymer at a temperature in the range of about 90 to about 160 vF.; forming the resulting References Cited in the fileof this patent UNITED STATES PATENTS 1,038,187 OBrien' Sept. 10, 1912 2,067,213 Snelling' Ian. 12, 1937 2,211,738 Cairns Aug. 13, 1940 2,616,787 "Whetstone Nov. 4, 1952 2,742,672 v Thomas Apr. 24, 1956 

1. A PROCESS FOR THE PRODUCTION OF A SOLID ROCKET PROPELLANT CHARGE COMPRISING FROM ABOUT 50 TO ABOUT 90 WEIGHT PERCENT AMMONIUM NITRATE AND FROM ABOUT 10 TO ABOUT 50 WEIGHT PERCENT OF A RUBBER BINDER WHICH COMPRISES DRYING THE AMMONIUM NITRATE AT A TEMPERATURE ABOVE 90*F. AND NOT SUBSTANTIALLY HIGHER THAN 200*F., INCORPORATING THE DRIED AMMONIUM NITRATE INTO SUBSTANTIALLY DRY, UNCURED RUBBER AT A TEMPERATURE IN THE RANGE ABOUT 90* AND 160*F., FORMING THE RESULTING MIXTURE INTO CHARGES, AND CURING THE CHARGES AT A TEMPERATURE IN THE RANGE OF ABOUT 175*F. TO ABOUT 180*F. SO THAT THE AMMONIUM NITRATE IS MAINTAINED AT A TEMPERATURE IN THE RANGE OF 90 TO 200*F. FROM THE DRYING STEP TO THE END OF THE CURING STEP. 